The Still and Box with an EMS plan 1 is the standard response if a flight declares a potential emergency. This was the response level given to the two recent flights with engines eating birds. If a flight has an emergency indicator light, smell of smoke in the cabin or other such related issues and requests to return to the airport they will usually set up an emergency response as a precaution.
From the Chicago Scanning homepage:
Still and Box: 2 Engines , 2 Trucks , 1 Batt. Chief ,
1 Squad (Still alarm response) then the BOX response of
2 Additional Engines,
1 Additional Truck ,
1 Tower Ladder ( 4 Truck Companies Total )
1 Ambulance( ALS )
2 Additional Batt. Chiefs,
1 Deputy District Chief
1 Office of Fire InspectionCompany ( OFI )
1 EMS Field Officer
Plus an EMS Plan 1 brings 5 additional ambulances
http://www.geocities.com/paulmachnick/CSCChiFire.htm
And a partridge in a pear tree ;)